1. Field of the Invention
The invention is directed to a method for operating an adjustable hydrostatic pump in accordance with the size of the opening of a throttle located in a hydraulic fluid delivery line connecting the pump and a hydraulic energy consuming device. The pressure drop, .DELTA.p, is measured at the throttle and the measured value is transmitted to a load-sensing controller which adjusts the displacement of the hydrostatic pump to a value at which the delivery pressure of the pump exceeds the pressure at the hydraulic energy consuming device by an amount which is determined by the system. The amount of the pressure difference is controllable. The invention includes a hydrostatic drive system which includes the adjustable hydrostatic pump and at least one hydraulic energy consuming device connected to the pump by a hydraulic fluid delivery line for performing the method of the invention. An adjustable throttle is located in the hydraulic fluid delivery line between the pump and the consuming device and a load-sensing controller is connected to the fluid delivery line on opposite sides of the throttle to reproduce the pressure drop at the throttle and to adjust the displacement of the pump.
Adjustable hydrostatic pumps which are operated on the load-sensing principle are known in an open circuit in the mobile sector (for example, in construction equipment). These pumps operate with a fixed pressure difference, .DELTA.p, and an arbitrarily controlled size of the opening of the throttle. The size of the opening of the throttle, which may be a part of a valve of a hydraulic energy consuming device, determines the speed of movement of the consuming device in the mobile sector.
The throttle is opened a small amount for slow movement of the hydraulic energy consuming device and pressure builds up in the hydraulic fluid delivery line between the throttle and the hydraulic energy consuming device which is initially stationary due to friction. The load-sensing controller maintains the pressure difference constant at the pre-set value and increases the delivery rate from the pump. Depending upon the coefficient of friction at the hydraulic energy consuming device, the delivery rate from the pump can increase to a level which is much higher than the level necessary to achieve the desired speed of movement of the hydraulic energy consuming device. Under these circumstances, an undesirable jerking motion of the hydraulic energy consuming device can take place when movement of the consuming device is initiated and the starting speed of the consuming device will be higher than the speed which has actually been set.
The throttle is opened a large amount for rapid movement of the hydraulic energy consuming device, and the pre-set pressure difference, .DELTA.p, drops considerably. This is desirable because the loss of power eliminated by the throttle becomes small at high volume flow rates (otherwise the result would be, for example, a 5% loss of power with an operating pressure of 400 bar and a pressure difference, .DELTA.p, of 20 bar). However, departure from the range of control of the load-sensing controller is disadvantageous.
The requirements are different when using an adjustable hydrostatic pump in the industrial sector (for example, a hydraulic press) wherein variable displacement pumps, i.e., pumps with which a particular control command which is always associated with a defined displacement of the pump and, thus, a particular speed of movement of the hydraulic energy consuming device, are normally used. Variable displacement pumps require movement feedback (or displacement feedback) and a special control valve system. Practically no loss of power occurs when variable displacement pumps are used. In the industrial sector, rapid movements of the hydraulic energy consuming device are desired. The use of a pump controlled by the load-sensing method in the industrial sector is not possible because the initial low speeds of movement of the hydraulic energy consuming device are associated with jerky movements and high speeds of movement of the hydraulic energy consuming device result in departures from the range of control.
Therefore, a pump manufacturer supplying both the mobile sector and the industrial sector must have a different pump with the same displacement available for each application.
Jerking movements with slow movements of the hydraulic energy consuming device can be largely prevented with a method of this type for operating an adjustable hydrostatic pump, such a method is also referred to as load-sensing control with fine-mode control and is disclosed in U.S. Pat. No. 4,976,106, which is incorporated herein by reference. Here, the pressure difference, .DELTA.p, is reduced by sensing a value in a load feedback line. As a result, the equilibrium at the load-sensing controller moves in a direction to reduce the displacement of the pump while the size of the opening of the throttle remains the same which results in increased fine controllability. When the hydraulic energy consuming device connected to the pump is manually controlled by a hand lever to vary the size of the opening of the throttle, half the quantity of the hydraulic fluid can be controlled over the entire adjusting range of the hand lever, for example, if the sensing valve is designed accordingly.
Thus, through the fine-mode control, the volume flow to the hydraulic energy consuming device is less when the pressure difference, .DELTA.p, is reduced, so that less movement occurs at the hydraulic energy consuming device with the same size opening of the throttle.
However, with a fast movement of the hydraulic energy consuming device, i.e., with the fine-mode control switched off, pumps under load-sensing control still have the drawback, already mentioned, that when the pressure difference, .DELTA.p, decreases, the load-sensing control, i.e., the actual volume flow control, no longer functions. The speed of movement of the hydraulic energy consuming device is not subject to disturbance control because the pump remains continuously set to maximum displacement. This means that fluctuating drive revolutions lead to fluctuating speeds of movement of the consuming device. This is a detriment to using a hydrostatic pump with load-sensing control in the industrial sector.